Coiled tubing has long been used in well operations in order to place desirable fluids such as acids, cement and the like in a well utilizing a relatively simple apparatus comprising a long length of tubing, often as long as 25,000 feet, wound onto a large spool or reel. In coiled tubing operations, tubing from the reel is fed into the wellbore utilizing an injector mechanism which is well known in the art. Fluids can be fed through a fitting on the tubing reel, through the tubing to a tool disposed on the inserted end of the coiled tubing within the well.
In recent years, it has become desirable to place electrically operated tools in the wellbore either alone or in combination with fluid treatment tools. For instance, logging operations can be conducted on coiled tubing which may also include means for injecting nitrogen gas into the wellbore in order to gauge flow. It is also advantageous to use coiled tubing to convey a logging tool within a highly deviated or horizontal wellbore where gravity wireline operations cannot be used to convey the logging tool to the appropriate wellbore location.
More recently, coiled tubing has been used in conjunction with a fluid operated downhole drilling motor to conduct slim hole drilling in vertical, deviated and horizontal directions. With such coiled tubing drilling operations, it is often desirable to have a device capable of sensing the direction of drilling and communicating this and other data to the operator on the surface so that alterations may be made to the direction of the drilling during the drilling process. It can be clearly seen that in order to employ logging tools as well as to operate steering sensors or tools in coiled tubing drilling operations, it is necessary to provide electrical communication between such tools and the surface through a cable disposed within the coiled tubing.
In the past, in order to install electrically conductive cable within coiled tubing, it has been necessary to deploy the coiled tubing in a relatively straight line such as deploying it into a vertical well or laying the coiled tubing horizontally such as on a straight road bed in order to feed the electrically conductive cable into the tubing. It is obvious that this is a time consuming, expensive and labor intensive solution to the problem of installing cable within tubing. In fact, the cost of installation in this manner often exceeds the cost of the cable itself by three to five times. Because of the high cost of cable installation, it has become economically necessary to maintain coiled tubing reels with cable installed dedicated to that use for the life of the reel. This is both expensive and wasteful of costly assets requiring either a large inventory of dedicated, cable-installed reels to be kept at widely spaced locations or only a few such dedicated reels which must be transported over long distances for use.
It is well known in the industry that it is theoretically possible to pump cable through coiled tubing while it is on the reel. The fluid friction drag force of the flowing fluid pulls the cable, overcoming friction forces between the cable and the coiled tubing. However, attempts to pump cable into coiled tubing on the reel have been thwarted by the cable tension induced by the cable seal through which the cable must be passed into the tubing. This cable tension generates a large frictional force as the cable is pulled tight within the tubing around the reel creating a capstan effect which prevents the cable from passing through the entire length of tubing on the reel.
A mechanical means to overcome the friction and pressure differential of the cable seal was disclosed in European patent application publication number 0565287A1. A mechanical pulling device is located within a sealed chamber connected to the coiled tubing and with the cable passing into the chamber through a seal. When used in conjunction with a fluid pump, cable can be injected into coiled tubing on the reel because the pulling device overcomes the friction of pulling the cable through the seal thereby avoiding the capstan effect, albeit with considerable mechanical complexity.